The mailer, first reported by the Des Moines Register, contains a letter signed by Santorum that says, "It truly frightens me to think what’ll happen if Mitt Romney is the nominee."

"My friend, Republicans and conservatives will be crippled by a nominee who presents zero contrast with Barack Obama on the major issues of this election," the letter says, repeating an attack on Romney that Santorum made constantly on the campaign trail. "Any attempt by Mitt Romney to attack President Obama's positions on healthcare, energy policy, social issues, or the economy will be easily neutralized."

Santorum dropped out of the presidential race a week ago and has yet to endorse anyone. During a Monday evening phone call with supporters, when backers asked who they ought to vote for in upcoming primaries, Santorum urged them to vote their conscience and pointedly did not endorse Romney.

"As far as how you vote, that’s up to you," Santorum told thousands of supporters on the conference call. "I haven’t supported any candidate at this point, so that’s really up to you."

Alice Stewart, a Santorum spokeswoman, said the mailer was approved, paid for and sent before Santorum dropped out of the race.